Non-aqueous electrolyte secondary cell

ABSTRACT

A non-aqueous electrolyte secondary cell is provided having enhanced safety against overcharge and reduced self-discharge. The non-aqueous electrolyte secondary cell includes: a positive electrode having a positive electrode active material; a negative electrode having a negative electrode active material; and a non-aqueous electrolyte containing a non-aqueous solvent and electrolytic salt. The non-aqueous solvent contains 20 to 80 volume % tertiary carboxylic acid ester represented by formula 1 at 25° C. and 1 atm. The non-aqueous electrolyte contains an alkylbenzene compound and/or a halogenated benzene compound. 
     
       
         
         
             
             
         
       
         
         
           
             where R1 to R4 each denote a straight-chained or branched alkyl group having 4 or less carbon atoms and may be the same or different.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The present invention relates to a technique to improve the safety ofnon-aqueous electrolyte secondary cells at the time of overcharge.

2) Description of the Related Art

Non-aqueous electrolyte secondary cells, for their high energy densityand high capacity, are widely used for the driving power sources ofmobile appliances.

Incidentally, non-aqueous electrolytes used in the non-aqueouselectrolyte secondary cells contain flammable organic solvents; when thecells should be overcharged, smoking or firing may result. This hascreated a need for a technique to improve the safety of non-aqueouselectrolyte secondary cells at the time of overcharge.

Patent documents 1 to 7 disclose techniques related to non-aqueouselectrolyte secondary cells.

Patent Document 1: WO02/015319.

Patent Document 2: WO02/059999.

Patent Document 3: Japanese Patent Application Publication No.2002-298909.

Patent Document 4: Japanese Patent Application Publication No.2003-59529.

Patent Document 5: Japanese Patent Application Publication No.2004-214189.

Patent Document 6: Japanese Patent Application Publication No.2004-327444.

Patent Document 7: Japanese Patent Application Publication No.10-112335.

However, these techniques cannot enhance the safety of the non-aqueouselectrolyte secondary cells at the time of overcharge withoutcompromising cell properties such as the cycle characteristic andpreservation characteristic.

SUMMARY OF THE INVENTION

In view of the foregoing and other problems, it is an object of thepresent invention to provide a technique to enhance the safety of thenon-aqueous electrolyte secondary cells at the time of overchargewithout compromising cell properties such as the cycle characteristicand preservation characteristic.

In order to accomplish the above and other objects, the presentinvention is configured as follows.

A non-aqueous electrolyte secondary cell includes: a positive electrodehaving a positive electrode active material; a negative electrode havinga negative electrode active material; and a non-aqueous electrolytecontaining a non-aqueous solvent and electrolytic salt. The non-aqueoussolvent contains 20 to 80 volume % tertiary carboxylic acid esterrepresented by formula 1 at 25° C. and 1 atm. The non-aqueouselectrolyte contains an alkylbenzene compound and/or a halogenatedbenzene compound.

where R1 to R4 each denote a straight-chained or branched alkyl grouphaving 4 or less carbon atoms and may be the same or different.

In order to enhance the safety of the cell at the time of overcharge (inorder to prevent smoking and the like at the time of overcharge at highrate), it is necessary to contain tertiary carboxylic acid esterrepresented by Formula 1 in the non-aqueous solvent at 20 volume %.However, containing the tertiary carboxylic acid ester in thenon-aqueous solvent at 20 volume % or more poses the problem offacilitated occurrence of self-discharge when the cell is left to stand.This decreases the cell voltage and increases the negative electrodepotential. The high potential negative electrode and the non-aqueouselectrolyte react with one another to decompose the non-aqueouselectrolyte, thereby degrading cell properties such as the cyclecharacteristic.

In the above configuration, in the non-aqueous electrolyte containingthe tertiary carboxylic acid ester, an alkylbenzene compound and/or ahalogenated benzene compound are added. The alkylbenzene compound andthe halogenated benzene compound serve to inhibit self-discharge of thenon-aqueous electrolyte secondary cells containing tertiary carboxylicacid ester. Thus, in the above configuration, the negative electrodepotential is prevented from increasing and the non-aqueous electrolyteis not decomposed, thereby preventing the degradation of the cellproperties. Thus, a non-aqueous electrolyte secondary cell securing bothsafety at the time of overcharge and good cell characteristics isrealized.

It should be noted that if the tertiary carboxylic acid ester iscontained at more than 80 volume %, self-discharge cannot be preventedsufficiently even if the alkylbenzene compound and the halogenatedbenzene compound are contained. In view of this, the upper limit of thecontent of the tertiary carboxylic acid ester is 80 volume %.

The halogenated benzene compound may have one hydrogen atom of thebenzene substituted with a halogen, or two or more hydrogen atomssubstituted with halogens. When two or more hydrogen atoms aresubstituted with halogens, the two or more halogens may be the same ordifferent. It is also possible that the rest of the hydrogen atoms ofthe halogenated benzene compound, which are not substituted withhalogen, be substituted with an alkyl group, a halogenated alkyl group,an alkoxy group, or the like.

The alkylbenzene compound may have one hydrogen atom of the benzenesubstituted with an alkyl group, or two or more hydrogen atoms of thebenzene substituted with alkyl groups. When two or more hydrogen atomsare substituted with alkyl groups, the two or more alkyl groups may bethe same or different. The alkyl substituent groups may have theirhydrogen atoms substituted with halogen. The alkyl substituent groupsalso may be either straight-chained, branched, or cyclic. It is alsopossible that the rest of the hydrogen atoms of the alkylbenzenecompound, which are not substituted with alkyl groups, be substitutedwith an alkoxy group or the like.

In the above configuration, the tertiary carboxylic acid ester may bemethyl trimethylacetate and/or ethyl trimethylacetate.

The methyl trimethylacetate (compound with methyl groups for all R1 toR4 in Formula 1) and the ethyl trimethylacetate (compound with an ethylgroup for R1 and methyl groups for R2 to R4 in Formula 1) are preferablyused in that they provide a large effect per unit mass.

In the above configuration, the alkylbenzene compound may be at leastone compound selected from the group consisting of cyclohexyl benzene,tert-amyl benzene, and tert-butyl benzene.

Cyclohexyl benzene, tert-amyl benzene, and tert-butyl benzene provide ahigh self-discharge preventing effect, and therefore at least one ofthem is preferably used.

In the above configuration, the content of the alkylbenzene compound maybe 0.3 to 5.0 mass parts out of 100 mass parts of a total mass of thenon-aqueous solvent and the electrolytic salt.

If the content of the alkylbenzene compound is less than 0.3 mass parts,a sufficient advantageous effect may not be obtained from thealkylbenzene compound, while if the content is more than 5.0 mass parts,the alkylbenzene compound may decrease the discharge capacity. In viewof this, the content of the alkylbenzene compound is preferably withinthe claimed range.

In the above configuration, the halogenated benzene compound may be atleast one compound selected from the group consisting of monofluorobenzene, monochloro benzene, 3-fluoroanisole, and 3,5-difluoroanisole.

Monofluoro benzene, monochloro benzene, 3-fluoroanisole, and3,5-difluoroanisole provide a high self-discharge preventing effect andcause no harm to the discharge capacity, and therefore at least one ofthem is preferably used.

In the above configuration, the content of the halogenated benzenecompound may be 0.2 to 4.5 mass parts out of 100 mass parts of a totalmass of the non-aqueous solvent and the electrolytic salt.

If the content of the halogenated benzene compound is less than 0.2 massparts, a sufficient advantageous effect may not be obtained from thehalogenated benzene compound, while if the content is more than 4.5 massparts, the halogenated benzene compound may decrease the dischargecapacity. In view of this, the content of the halogenated benzenecompound is preferably within the claimed range.

It should be noted that although the non-aqueous electrolyte may containknown additives such as vinylene carbonate, vinyl ethylene carbonate,and vinyl acetate, they are treated as additives and will not beincluded in the total mass of the non-aqueous solvent and theelectrolytic salt.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described below.It will be appreciated that the present invention will not be limited tothe examples below, and that any practice of the invention with suitableamendments is possible without departing from the scope of theinvention.

Embodiments Preparation of the Positive Electrode

Ninety-five mass parts of lithium cobalt composite oxide serving as apositive electrode active material, 2 mass parts of acetylene serving asa conducting agent, 3 mass parts of polyvinylidene fluoride (PVdF)serving as a binding agent, and N-methyl-2-pyrrolidone (NMP) are mixedtogether, thus preparing a positive electrode active material slurry.This positive electrode active material slurry is applied to bothsurfaces of a positive electrode current collector (12 μm thick) made ofaluminum by doctor blading, followed by drying to remove the solvent(NMP), which is necessary for preparation of the slurry. Then, the driedelectrode plate is rolled to a thickness of 120 μm and cut to apredetermined size, thus completing a positive electrode.

<Preparation of the Negative Electrode>

Ninety-five mass parts of a negative electrode active material made ofartificial graphite (d(002) value: 0.336 nm) with its surfacenon-crystallized, 5 mass parts of polyvinylidene fluoride (PVdF) servingas a binding agent, and N-methyl-2-pyrrolidone are mixed together, thuspreparing a negative electrode active material slurry. This negativeelectrode active material slurry is applied to both surfaces of anegative electrode current collector (8 μm thick) made of copper,followed by drying to remove the solvent (NMP), which is necessary forpreparation of the slurry. Then, the dried electrode plate is rolled toa thickness of 130 μm and cut to a predetermined size, thus completing anegative electrode.

<Preparation of the Electrode Assembly>

The positive and negative electrodes were wound with a separator (16 μmthick) made of finely porous film of polyolefin, and then pressed, thuspreparing a flatly wound electrode assembly.

<Preparation of the Non-aqueous Electrolyte>

Methyl trimethylacetate (MTMA) represented by Formula 2 and ethylenecarbonate (EC) are mixed in a volume ratio of 80:20 at 25° C. and 1 atmto form a non-aqueous solvent. Next, LiPF₆ serving as an electrolytesalt is dissolved at a rate of 1.0 M (mol/litter) in this mixture so asto prepare a non-aqueous electrolytic solution. To 100 mass parts of thenon-aqueous electrolytic solution, a benzene compound is added, thuspreparing a non-aqueous electrolyte.

where R1 to R4 are all methyl groups.

<Assembly of the Cell>

A commercial aluminum laminate is prepared. This aluminum laminatematerial is folded to form a bottom portion and a cup-form electrodeassembly storage space. Then, this flatly wound electrode assembly isinserted into the storage space.

After the flatly wound electrode assembly is inserted into the storagespace, the outer casing is depressurized to impregnate a separator withthe non-aqueous electrolyte. Then the opening portion of the outercasing is sealed, thus preparing a non-aqueous electrolyte secondarycell.

Examples 1 to 45, Comparative Examples 1 to 25

Cells according to examples 1 to 45 and comparative examples 1 to 25were prepared in the same manner as in the above embodiment except thatthe kind of the non-aqueous solvent, the volume mixture ratio of thenon-aqueous solvent, the kind of the benzene compound, and the contentof the benzene compound were varied as shown in Tables 1 to 6.

[Measurement of the Initial Capacity]

Cells were prepared in the same manner as in examples 1 to 45 andcomparative examples 1 to 25. These cells were charged at a constantcurrent of 0.5 It (390 mA) to a voltage of 4.2 V and then at a constantvoltage of 4.2 V for 5 hours in total. Then, the cells were dischargedat a constant current of 0.5 It (390 mA) to a voltage of 2.75 V. Thedischarge capacity of each cell was measured and assumed an initialcapacity. The charge and discharge were carried out under a condition of23° C. The results are shown in Tables 1 to 6.

[Self-Discharge Property Test]

Cells were prepared in the same manner as in examples 1 to 45 andcomparative examples 1 to 25. These cells were charged under the sameconditions as in the measurement of the initial capacity, and thendischarged. The discharged cells were left to stand at 23° C. for 90days, and the voltage of each cell was measured before and after eachcell was left to stand. The difference (ΔV) of cell voltage before andafter each cell was left to stand is shown in Tables 1 to 6.

[Measurement of the 60° C. Cycle Characteristics]

Cells were prepared in the same manner as in examples 4 and 31 to 45 andcomparative examples 24 and 25. These cells were subjected to 500 cyclesof charge and discharge under the following conditions.

i) The cells were charged at a constant current of 1.0 It (780 mA) to avoltage of 4.2 V and then at a constant voltage of 4.2 V for 3 hours intotal.

ii) The cells were discharged at a constant current of 1.0 It (780 mA)to a voltage of 2.75 V.

iii) Return to i).

The cycle characteristic of each cell was calculated from the followingformula. The charge-discharge cycles were carried out under a conditionof 60° C. The results are shown in Table 6.

60° C. cycle characteristic (%)=500th cycle discharge capacity/1st cycledischarge capacity×100

[Overcharge Safety Test]

Cells were prepared in the same manner as in examples 4, 11 to 14, and18 to 30 and comparative examples 20 and 23.

i) These cells were charged at a constant current of 0.6 It (468 mA) toa voltage of 12.0 V and then at a constant voltage of 12.0 V for 15hours in total. The cases where smoking occurred during this overchargewere not further tested, and the current value at this time was assumeda current limit value.

ii) For the cases where no smoking occurred, respective new cells wereprepared under the same conditions. The new cells were subjected to thesame test as i) except that the constant current during the charge wasraised to 0.1 It (78 mA).

iii) The cases where no smoking occurred were subjected to a test with aconstant current value raised in the ii) manner during the charge untilsmoking was observed.

A maximum current rate of each cell at which no smoking was observed wasestimated as a current limit value, and the results are shown in Tables4 and 5. The test was carried out at 23° C.

TABLE 1 Non-aqueous Benzene Initial Voltage drop solvent compoundcapacity amount Kind Volume ratio Kind Mass % (mAh) (ΔV) ComparativeEC/MTMA 30/70 — — 784 0.65 Example 1 Example 1 EC/MTMA 30/70 Monofluoro0.2 785 0.16 benzene Example 2 EC/MTMA 30/70 Monofluoro 0.5 785 0.08benzene Example 3 EC/MTMA 30/70 Monofluoro 1.0 782 0.07 benzene Example4 EC/MTMA 30/70 Monofluoro 2.0 784 0.08 benzene Example 5 EC/MTMA 30/70Monofluoro 4.0 787 0.06 benzene Example 6 EC/MTMA 30/70 Monofluoro 4.5780 0.05 benzene Example 7 EC/MTMA 30/70 Monofluoro 5.0 769 0.05 benzeneComparative EC/DEC 30/70 — — 785 0.07 Example 2 Comparative EC/DEC 30/70Monofluoro 2.0 780 0.09 Example 3 benzene

TABLE 2 Voltage Non-aqueous Benzene Initial drop solvent compoundcapacity amount Kind Volume ratio Kind Mass % (mAh) (ΔV) ComparativeEC/MTMA 30/70 — — 784 0.65 Example 1 Example 8 EC/MTMA 30/70 tert-amyl0.3 788 0.21 benzene Example 9 EC/MTMA 30/70 tert-amyl 0.5 784 0.12benzene Example 10 EC/MTMA 30/70 tert-amyl 1.0 786 0.05 benzene Example11 EC/MTMA 30/70 tert-amyl 2.0 781 0.06 benzene Example 12 EC/PC/MTMA30/5/65  tert-amyl 2.0 788 0.05 benzene Example 13 EC/PC/MTMA 30/10/60tert-amyl 2.0 783 0.05 benzene Example 14 EC/PC/MTMA 30/20/50 tert-amyl2.0 785 0.05 benzene Example 15 EC/MTMA 30/70 tert-amyl 4.0 782 0.05benzene Example 16 EC/MTMA 30/70 tert-amyl 5.0 779 0.05 benzene Example17 EC/MTMA 30/70 tert-amyl 5.5 762 0.05 benzene Comparative EC/DEC 30/70— — 785 0.07 Example 2 Comparative EC/DEC 30/70 tert-amyl 1.0 782 0.08Example 4 benzene Comparative EC/DEC 30/70 tert-amyl 2.0 789 0.07Example 5 benzene Comparative EC/PC/DEC 30/5/65  tert-amyl 2.0 784 0.06Example 6 benzene Comparative EC/PC/DEC 30/10/60 tert-amyl 2.0 781 0.06Example 7 benzene Comparative EC/PC/DEC 30/20/50 tert-amyl 2.0 782 0.06Example 8 benzene

TABLE 3 Non-aqueous Benzene Initial Voltage drop solvent compoundcapacity amount Kind Volume ratio Kind Mass % (mAh) (ΔV) ComparativeEC/DEC/MTMA 30/60/10 — — 784 0.11 Example 9 Comparative EC/DEC/MTMA30/50/20 — — 783 0.47 Example 10 Comparative EC/DEC/MTMA 30/40/30 — —783 0.54 Example 11 Comparative EC/DEC/MTMA 30/50/20 — — 785 0.61Example 12 Comparative EC/EMC/MTMA 30/20/50 — — 788 0.60 Example 13Comparative EC/PC/MTMA 30/5/65  — — 786 0.66 Example 14 ComparativeEC/PC/MTMA 30/10/60 — — 783 0.57 Example 15 Comparative EC/PC/MTMA30/20/50 — — 781 0.55 Example 16 Comparative EC/MTMA 30/70 — — 784 0.65Example 1 Comparative EC/ETMA 30/70 — — 781 0.61 Example 17 ComparativeEC/MTMA 20/80 — — 782 0.96 Example 18 Comparative EC/MTMA 10/90 — — 7741.37 Example 19

TABLE 4 Voltage Non-aqueous Benzene Initial drop Current solventcompound capacity amount limit Kind Volume ratio Kind Mass % (mAh) (ΔV)value (It) Comparative EC/DEC/MTMA 30/60/10 Monofluoro 2.0 785 0.08 0.7Example 20 benzene Example 18 EC/DEC/MTMA 30/50/20 Monofluoro 2.0 7830.06 1.3 benzene Example 19 EC/DEC/MTMA 30/40/30 Monofluoro 2.0 781 0.071.5 benzene Example 20 EC/DEC/MTMA 30/20/50 Monofluoro 2.0 782 0.07 1.6benzene Example 21 EC/EMC/MTMA 30/20/50 Monofluoro 2.0 784 0.10 1.6benzene Example 22 EC/PC/MTMA 30/20/50 Monofluoro 2.0 780 0.11 2.1benzene Example 4 EC/MTMA 30/70 Monofluoro 2.0 784 0.08 2.3 benzeneExample 23 EC/ETMA 30/70 Monofluoro 2.0 781 0.11 2.2 benzene Example 24EC/MTMA 20/80 Monofluoro 2.0 780 0.18 2.3 benzene Comparative EC/MTMA10/90 Monofluoro 2.0 770 1.09 2.3 Example 21 benzene

TABLE 5 Voltage Non-aqueous Benzene Initial drop Current solventcompound capacity amount limit Kind Volume ratio Kind Mass % (mAh) (ΔV)value (It) Comparative EC/DEC/MTMA 30/60/10 tert-amyl 2.0 784 0.10 0.7Example 22 benzene Example 25 EC/DEC/MTMA 30/50/20 tert-amyl 2.0 7830.07 1.3 benzene Example 26 EC/DEC/MTMA 30/40/30 tert-amyl 2.0 783 0.061.5 benzene Example 27 EC/DEC/MTMA 30/20/50 tert-amyl 2.0 789 0.06 1.7benzene Example 28 EC/EMC/MTMA 30/20/50 tert-amyl 2.0 786 0.07 1.7benzene Example 11 EC/MTMA 30/70 tert-amyl 2.0 781 0.06 2.3 benzeneExample 12 EC/PC/MTMA 30/5/65  tert-amyl 2.0 788 0.05 2.3 benzeneExample 13 EC/PC/MTMA 30/10/60 tert-amyl 2.0 783 0.05 2.1 benzeneExample 14 EC/PC/MTMA 30/20/50 tert-amyl 2.0 785 0.05 2.1 benzeneExample 29 EC/MTMA 30/70 tert-amyl 2.0 781 0.08 2.2 benzene Example 30EC/MTMA 30/80 tert-amyl 2.0 782 0.05 2.3 benzene Comparative EC/MTMA10/90 tert-amyl 2.0 784 1.42 2.3 Example 23 benzene

TABLE 6 Non-aqueous Benzene Voltage 60° C. solvent compound Initial dropcycle Volume Mass capacity amount characteristic Kind ratio Kind % (mAh)(ΔV) (%) Comparative EC/MTMA 30/70 Benzene 2.0 787 0.71 33 Example 24Comparative EC/MTMA 30/70 Anisole 2.0 784 0.92 51 Example 25 Example 4EC/MTMA 30/70 Monofluoro 2.0 784 0.08 83 benzene Example 31 EC/MTMA30/70 1,3-difluoro 2.0 787 0.11 72 benzene Example 32 EC/MTMA 30/701,3,5-trifluoro 2.0 782 0.09 75 benzene Example 33 EC/MTMA 30/70Monochloro 2.0 783 0.13 82 benzene Example 34 EC/MTMA 30/70 Monofluoro2.0 780 0.07 72 biphenyl Example 35 EC/MTMA 30/70 Fluorocyclohexyl 2.0782 0.05 77 benzene Example 36 EC/MTMA 30/70 2-fluoroanisole 3.0 7860.07 74 Example 37 EC/MTMA 30/70 3-fluoroanisole 3.0 781 0.07 86 Example38 EC/MTMA 30/70 2,4-difluoroanisole 3.0 784 0.04 73 Example 39 EC/MTMA30/70 3,5-difluoroanisole 2.0 783 0.04 84 Example 40 EC/PC/MTMA 30/20/503,5-difluoroanisole 2.0 782 0.04 84 Example 11 EC/MTMA 30/70 tert-amylbenzene 2.0 781 0.06 65 Example 41 EC/MTMA 30/70 tert-butyl benzene 2.0785 0.08 67 Example 42 EC/MTMA 30/70 Cyclohexyl 2.0 785 0.07 68 benzeneExample 43 EC/MTMA 30/70 Toluene 2.0 780 0.12 66 Example 44 EC/MTMA30/70 Ethylbenzene 2.0 783 0.11 63 Example 45 EC/MTMA 30/70n-butylbenzene 2.0 788 0.13 65

In Tables 1 to 6, the meanings of the abbreviations in the section of“non-aqueous solvent” are as follows:

EC: ethylene carbonate

PC: propylene carbonate

DEC: diethyl carbonate

EMC: ethylmethyl carbonate

MTMA: methyl trimethylacetate

ETMA: ethyl trimethylacetate

Table 3 shows that in comparative examples 1, 2, and 9 to 19, whichcontain no benzene compounds such as alkylbenzene compounds andhalogenated benzene compounds, comparative examples 1 and 10 to 19,which contain 20 volume % or more tertiary carboxylic acid ester (methyltrimethylacetate (MTMA) and ethyl trimethylacetate (ETMA)), have voltagedrops of 0.47 to 1.37 V, which are significantly larger than a 0.11 Vvoltage drop for comparative example 9, which has a methyltrimethylacetate (MTMA) content of 10 volume %, and a 0.07 V voltagedrop for comparative example 2, which contains no (0 volume %) methyltrimethylacetate (MTMA).

A possible explanation for this is as follows. In the case of containing20 volume % or more tertiary carboxylic acid ester, self-dischargeeasily occurs, though a reason therefor is yet to be revealed. Thus, thecell voltage greatly decreases (i.e., the voltage drop increases).

Table 1 shows that examples 1 to 7, which contain 70 volume % methyltrimethylacetate (MTMA) and have a monofluoro benzene content of 0.2mass % or more, have voltage drops of 0.05 to 0.16 V, which are superiorto a 0.65 V voltage drop for comparative example 1, which contains 70volume % methyl trimethylacetate (MTMA) and contains no monofluorobenzene.

A possible explanation for this is that the halogenated benzenecompounds such as monofluoro benzene provide the effect of preventingthe cell containing methyl trimethylacetate (MTMA) fromself-discharging, thereby lessening the voltage drop.

Table 1 also shows that example 7, which contains 70 volume % methyltrimethylacetate (MTMA) and has a monofluoro benzene content of 5.0 mass%, has an initial capacity of 769 mAh, which is smaller than 780 to 787mAh initial capacities for examples 1 to 6, which contain 70 volume %methyl trimethylacetate (MTMA) and have a monofluoro benzene content of0.2 to 4.5 mass %.

A possible explanation for this is as follows. Containing a large amountof halogenated benzene compound such as monofluoro benzene decreases thelithium ion conductivity of the non-aqueous electrolyte, resulting in adecrease in discharge capacity. In view of this, the content of thehalogenated benzene compound such as monofluoro benzene is preferably0.2 to 4.5 mass %.

Table 2 shows that examples 8 to 17, which contain 50 to 70 volume %methyl trimethylacetate (MTMA) and have a tert-amyl benzene content of0.3 mass % or more, have voltage drops of 0.05 to 0.21 V, which aresuperior to a 0.65 V voltage drop for comparative example 1, whichcontains 70 volume % methyl trimethylacetate (MTMA) and contains notert-amyl benzene.

A possible explanation for this is as follows. The alkylbenzenecompounds such as tert-amyl benzene provide the effect of preventing thecell containing methyl trimethylacetate (MTMA) from self-discharging,thereby lessening the voltage drop.

Table 2 also shows that example 17, which contains 70 volume % methyltrimethylacetate (MTMA) and has a tert-amyl benzene content of 5.5 mass%, has an initial capacity of 762 mAh, which is smaller than 779 to 788mAh initial capacities for examples 8 to 16, which contain 50 to 70volume % methyl trimethylacetate (MTMA) and have a tert-amyl benzenecontent of 0.3 to 5.0 mass %.

A possible explanation for this is as follows. Containing a large amountof alkylbenzene compound such as tert-amyl benzene decreases the lithiumion conductivity of the non-aqueous electrolyte, resulting in a decreasein discharge capacity. In view of this, the content of the alkylbenzenecompound is preferably 0.3 to 5.0 mass %.

Tables 4 and 5 show that comparative examples 20 and 22, which have amethyl trimethylacetate (MTMA) content of 10 volume % or less, have acurrent limit of 0.7 It, which is inferior to 1.3 to 2.3 It currentlimits for examples 4 and 11 to 14, which have a methyl trimethylacetate(MTMA) content of 20 volume % or more.

A possible explanation for this is that if the content of the tertiarycarboxylic acid ester is less than 20 volume %, the security of the cellat the time of overcharge cannot be sufficiently improved.

Tables 4 and 5 also show that comparative examples 21 and 23, whichcontain monofluoro benzene or tert-amyl benzene and have a methyltrimethylacetate (MTMA) content of 90 volume %, have voltage drops of1.09 V and 1.42 V, which are inferior to 0.05 to 0.18 V voltage dropsfor examples 4, 11 to 14, and 18 to 30, which contain monofluoro benzeneor tert-amyl benzene and have a methyl trimethylacetate content of 20 to80 volume %.

A possible explanation for this is as follows. If a large amount oftertiary carboxylic acid ester such as methyl trimethylacetate iscontained, self-discharge cannot be sufficiently prevented even with theaddition of the halogenated benzene compound and the alkylbenzenecompound, resulting in a drop in voltage. In view of this, the upperlimit of the tertiary carboxylic acid ester is preferably 80 volume %.

Table 6 shows that a sufficient advantageous effect can be obtainedusing any of the various halogenated benzene compounds and alkylbenzenecompounds, and that no advantageous effects can be obtained from benzenecompounds without halogen and alkyl groups, such as anisole (comparativeexample 24) and benzene (comparative example 25).

Table 6 also shows that the cases of using monofluoro benzene,monochloro benzene, 3-fluoroanisole, and 3,5-difluoroanisole (examples4, 33, 37, 39, and 40) have 60° C. cycle characteristics of 82 to 86%,which are superior to 72 to 77% 60° C. cycle characteristics for thecases of using other halogenated benzene compounds (examples 31, 32, 34,36, and 38). In view of this, when a halogenated benzene compound isused, at least one compound selected from the group consisting ofmonofluoro benzene, monochloro benzene, 3-fluoroanisole, and3,5-difluoroanisole is preferably used. Also, these results show that asimilar advantageous effect can be obtained from the case where ahydrogen atom of the benzene is substituted with an alkoxy group (i.e.,halogenated anisole is used).

Table 6 also shows that the cases of using cyclohexyl benzene, tert-amylbenzene, and tert-butyl benzene (examples 11, 41, and 42) have voltagedrops of 0.06 to 0.08 V, which are superior to 0.11 to 0.13 V voltagedrops for the cases of using other alkylbenzene compounds (examples 43to 45). In view of this, when an alkylbenzene compound is used, at leastone compound selected from the group consisting of cyclohexyl benzene,tert-amyl benzene, and tert-butyl benzene is preferably used.

INDUSTRIAL APPLICABILITY

As has been described hereinbefore, the present invention realizes anon-aqueous electrolyte secondary cell that is highly safe at the timeof overcharge and has minimized self-discharge. Therefore, theindustrial applicability of the present invention is considerable.

1. A non-aqueous electrolyte secondary cell comprising: a positiveelectrode having a positive electrode active material; a negativeelectrode having a negative electrode active material; and a non-aqueouselectrolyte containing a non-aqueous solvent and electrolytic salt,wherein: the non-aqueous solvent contains 20 to 80 volume % tertiarycarboxylic acid ester represented by formula 1 at 25° C. and 1 atm, andthe non-aqueous electrolyte contains an alkylbenzene compound and/or ahalogenated benzene compound.

where R1 to R4 each denote a straight-chained or branched alkyl grouphaving 4 or less carbon atoms and may be the same or different.
 2. Thenon-aqueous electrolyte secondary cell according to claim 1, wherein thetertiary carboxylic acid ester is methyl trimethylacetate and/or ethyltrimethylacetate.
 3. The non-aqueous electrolyte secondary cellaccording to claim 1, wherein the alkylbenzene compound is at least onecompound selected from the group consisting of cyclohexyl benzene,tert-amyl benzene, and tert-butyl benzene.
 4. The non-aqueouselectrolyte secondary cell according to claim 1, wherein the content ofthe alkylbenzene compound is 0.3 to 5.0 mass parts out of 100 mass partsof a total mass of the non-aqueous solvent and the electrolytic salt. 5.The non-aqueous electrolyte secondary cell according to claim 1, whereinthe halogenated benzene compound is at least one compound selected fromthe group consisting of monofluoro benzene, monochloro benzene,3-fluoroanisole, and 3,5-difluoroanisole.
 6. The non-aqueous electrolytesecondary cell according to claim 1, wherein the content of thehalogenated benzene compound is 0.2 to 4.5 mass parts out of 100 massparts of a total mass of the non-aqueous solvent and the electrolyticsalt.